Nitrogen oxides, such as NO and NO2 (collectively referred to as NOx), generated in the high temperature and high pressure conditions of an internal combustion engine, may constitute a large percentage of total exhaust emissions. Accordingly, engine exhaust systems may utilize selective catalytic reduction (SCR) to reduce the NOx species to diatomic nitrogen and water.
A variety of SCR catalysts have been developed including base metal catalysts and zeolite catalysts. However, the inventors herein have recognized several issues with such catalysts. Specifically, the catalysts may have limited operating temperature ranges, varying thermal durability, and may suffer from ammonia slip. As one example, copper-exchanged zeolites may efficiently reduce NOx at lower temperatures. However, at higher temperatures, they may oxidize the reducing agent leading to poor NOx conversion.
In one example, some of the above issues may be addressed by a system for a vehicle including an engine having an exhaust, the system comprising a NOx reducing system coupled to the engine exhaust. The NOx reducing system may include a catalytic unit with a first zeolite catalyst with a first NOx conversion performance in a first temperature range and a second NOx conversion performance, lower than said first NOx conversion performance, in a second temperature range. The catalytic unit may also include a second zeolite catalyst with a third NOx conversion performance, lower than said first NOx conversion performance, in the first temperature range and a fourth NOx conversion performance, higher than said second and third NOx conversion performances in the second temperature range, said first temperature range being higher than said second temperature range. The system may further comprise a controller configured to adjust an amount of reducing agent added to the NOx reducing system responsive to a temperature of the catalytic unit.
In one example, the catalytic unit may include a first Fe-exchanged zeolite catalyst with a (first) higher NOx conversion performance in the first higher temperature range. However, in a second lower temperature range, the Fe-exchanged zeolite catalyst may have a (second) substantially lower performance. The catalytic unit may also include a second Cu-exchanged zeolite catalyst with a (third) lower performance in the aforementioned higher temperature range, but may be configured to have a (fourth) higher performance in the aforementioned lower temperature range. In other words, the first zeolite catalyst may be configured with a higher optimal operating temperature range while the second zeolite catalyst may be configured with a lower optimal operating temperature range.
In this way, by combining catalysts with differing NOx conversion performances in differing operating temperature ranges, and by adjusting reducing agent delivery responsive to catalyst temperature, a catalyst combination with a substantially broader operating temperature range and a significantly improved NOx conversion performance over that broad operating temperature range may be generated. By using such an improved catalyst combination in a NOx reducing system, the quality of exhaust emissions may be improved. Further, by adjusting an amount of reducing agent delivered, for example by increasing reducing agent delivery at higher temperatures, increased NOx performance over the broader temperature range can be achieved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.